This activity engages students in the new definitions of ?planet,? ?dwarf planet,? and ?asteroid? through the use of direct vocabulary instruction. The publicity generated by the International Astronomical Union?s definition of Pluto as a dwarf planetAU in 2006 created a teachable moment for schools around the world to consider the definitions of these concepts of ?planet,? ?dwarf planet,? and ?asteroid? with the potential for a better understanding of our Solar System and how discoveries necessitate a change in the language we use to talk about it.